Move ListImplementation from coreimpl to core, as a private member.

lib/coreimpl/list.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/**
- * A [List] is an indexable collection with a length. It can be of
- * fixed size or extendable.
- */
-class ListImplementation<E> {
-  /**
-   * Factory implementation of List().
-   *
-   * Creates a list of the given [length].
-   */
-  external factory List([int length]);
-
-  /**
-   * Factory implementation of List.from().
-   *
-   * Creates a list with the elements of [other]. The order in
-   * the list will be the order provided by the iterator of [other].
-   */
-  factory List.from(Iterable<E> other) {
-    // TODO(ajohnsen): Make external once the vm can handle it, so we don't
-    // lose generic type information.
-    // Issue: #4727
-    return _from(other);
-  }
-
-  external static List _from(Iterable other);
-}